WP Smush- not compressing every image

Hi,

I downloaded WP Smush some time ago and yet am really frustrated with it. Every time I go to it and have not changed anything (or even added new images) it always changes the number of images that are still outstanding to be smushed. It is also telling me I have 50 files that can't be smushed due to being the wrong file format, however all the files it lists are either a JPEG, GIF or PNG, so I have no idea why these are a problem.

I continuously get pulled up by Google Speed Test and GT Metrix that I'm not serving all optimised and scaled images. But this is exactly what I pay for!

I'm getting really frustrated and ready to pull the plug!

All help would be greatly appreciated.